It is well known that siloxane copolymers can be prepared to provide an advantageous balance of properties and economics. In comparison with mixtures of homopolymers, the copolymers are generally more effective in producing any desired property, and the tendency to separate on a macroscopic scale is avoided. Copolymers of diorganopolysiloxanes can be prepared by mixing and polymerizing the respective cyclic siloxanes, but if one of them has a silicon-bonded aliphatic or haloaliphatic radical of three carbon atoms or greater, then only up to 20 mol % of the other co-monomer, which does not include such a structural limitation can be incorporated. Johannson, U.S. Pat. No. 3,002,951, illustrates the problem and the limitation. Johannson discloses that if a cyclic trisiloxane, having a 3 or more carbon silicon-bonded organo substituent, is reacted in admixture with another cyclic diorganosiloxane compound, in the presence of a strong alkali catalyst, only up to 10 mol % of the co-monomer will polymerize. It is stated in Johannson that if one starts with cyclic tetrasiloxanes and subjects them to alkaline polymerization conditions, that no apparent polymerization occurs.
Surprising, now, it has been discovered that under some conditions, the cyclic tetramers will readily homopolymerize, and also copolymerize. Most surprisingly, copolymerization with the cyclic tetramer will occur over a wide variety of composition ranges, and is not limited to the 10 mol % maximum found to be limiting with the cyclic trimer as reported by Johannson.
In general processes for producing diorganopolysiloxane gums and oils, diorganochlorosilanes of high purity are used as starting materials, and hydrolyzed in water at about 20.degree.-25.degree. C. The hydrolyzate is separated from the water and then a catalyst, such as potassium hydroxide, is added. The mixture is heated at temperatures of above 100.degree. C. for from 1 hour to 8 hours, the cyclic trisiloxanes, cyclic tetrasiloxanes, cyclic pentasiloxanes and the like are removed by distillation. Cracking of the hydrolysate in this fashion results in the production of a large amount of cyclic siloxanes from the hydrolyzate, and the most predominant fraction is the cyclic tetrasiloxane. It is known that methyl-, vinyl-, or phenyl-substituted such tetrasiloxanes in pure form can be equilibrated with a catalyst, such as potassium hydroxide to form oils and gums.
At equilibration, however, some of the already formed diorganopolysiloxane oil or gum also breaks down and reforms cyclic tetrasiloxane. Thus, at most only 85% of the original cyclic tetrasiloxanes can be converted to the desired diorganopolysiloxane gum or oil, with the other 15% by weight of the cyclic polysiloxanes being present in equilibrium with diorganopolysiloxane gum or oil. Copolymerization also results in such equilibria.
At equilibration, then, the catalyst can be neutralized and the volatiles removed to produce the desired diorganopolysiloxane oil or gum. This procedure can be used to obtain a diorganopolysiloxane copolymer oil or gum of anywhere from 30 to 200,000,000 centipoise at 25.degree. C.
Pierce et al, U.S. Pat. No. 2,979,519, disclose that because the cyclic tetrasiloxanes cannot be homopolymerized, it was surprising to find that cyclic trisiloxanes could be. However, a difficulty with the Pierce et al procedure is that the cyclic trisiloxanes themselves are formed in low yield during the initial cracking of the hydrolyzate with potassium hydroxide. This necessitates the use of energy consuming procedures, such as high reflux ratios to maximize the yield of cyclic trisiloxanes from the cracking process. The tetramers, on the other hand, form more readily. Thus, the process for forming polymers from cyclic trisiloxanes is more expensive than it would be if cyclic tetrasiloxanes could be used.
It has now been unexpectedly found that at certain low temperature ranges which were not envisioned previously and in the presence of certain selected catalysts, cyclic tetrasiloxanes can be mixed with cyclic co-monomers and the mixture can be equilibrated to form copolymers in relatively high yield, even where one of the organo substituent groups attached to the silicon atom is aliphatic or haloaliphatic of 3 carbon atoms, or more, such as the --CH.sub.2 CH.sub.2 R.sup.7 radical, where R.sup.7 is a perfluoroalkyl radical. Such cyclic tetrasiloxanes in admixture with other cyclics can be equilibrated at relatively high yields to produce low molecular weight copolymer oils for use as plasticizers or defoamers or high molecular weight diorganopolysiloxane copolymer gums suitable for forming heat vulcanizable silicone rubber compositions.
The discovery has prime importance to the use of methyl-3,3,3-trifluoromethylsiloxane cyclic tetramer because the conditions allow the copolymerization of the fluorosilicone tetramer with dimethyl tetramer (or dimethyl trimer) in the range of 30 to 98 mol % fluorosilicone. The ability to produce such copolymers is an advance in the art since, if the full solvent resistance properties of the fluorosilicone were not needed, a blend of fluorosilicone polymer with methyl polymers would be necessary. However, a copolymer is more efficient in solvent resistance than a blend at the same fluorosilicone content. In addition, while stable blends of fluorosilicone and methyl polymers can be made for high viscosity gums, it is impossible to make stable blends of lower viscosity oils, such as would be used in room temperature vulcanizing and fluid products, because the incompatibility of the fluorosilicone oil with the methyl oil will cause separation of these two components. The best balance of economy and solvent resistance is achieved with a copolymer.
It is, accordingly, a principal object of the present invention to provide a process for producing diorganopolysiloxane copolymer gums or oils in high yield, using a cyclic tetramer wherein at least one of the organo groups attached to the silicon atom is aliphatic or haloaliphatic of at least three carbon atoms or more.
Another object of the present invention is to provide low molecular weight diorganopolysiloxane copolymer oils and gums having a viscosity from 30 to 200,000,000 centipoise at 25.degree. C., and wherein at least one of the organo groups attached to the silicon atoms in one of the co-monomers is aliphatic or haloaliphatic of 3 carbon atoms or more, by a process comprising equilibrating cyclic tetrasiloxanes in admixture of cyclic co-monomers at low temperatures in the presence of certain select catalysts.
A further object of the present invention to provide diorganopolysiloxane copolymer oils or gums having a viscosity from 30 to 200,000,000 centipoise at 25.degree. C., wherein at least one of the co-monomers comprises 30 to 85 mol % of the copolymer units, and includes organo groups attached to the silicon atoms having at least 3 carbon atoms, and particularly, a --CH.sub.2 CH.sub.2 R.sup.7 substituent group, where R.sup.7 is perfluoroalkyl, using a cyclic tetrasiloxane as a co-monomer.